Long-fibered solids, for example, ropes, nylon stockings and similar items which may be contained in sewage, have a tendency that the part which first emerges from the impeller blade of a pump having a single blade impeller wraps itself round the impeller so that the emerging part again enters the impeller channel from the other side of the blade flank. This forms a loop with the part still remaining in the impeller channel which disturbs the fluid flow and results in other solid parts becoming stuck in the impeller, hence leading to the blockage of the pump.
Known pumps for such deposited solids have sought to overcome the problem by placing above the blade discharge edge a knife for cutting open the suspended long-fibered solids. A disadvantage of this arrangement is that solids which cannot be cut by the blade jam between the impeller blade and the housing wall and block the impeller. Further, as the knife is directly exposed to the entire conveying stream with all its deposited abrasive solids, it quickly leads to the knife being blunted unnecessarily.